The invention relates to a positioning system for a measuring device adapted for being carried on a satellite and particularly to an optical instrument or to an instrument for measuring inertial forces and moments, such as, an accelerometer.
The invention relates in particular to instruments carried on a satellite in which the requirements for alignment accuracy or positioning of the instrument are so high that the placement and positional control of the satellite can no longer provide the measurement signals necessary for this from external sensors, but rather, the signals must be provided directly from the very precise measurements of the measuring instrument for its own control.
Most scientific satellites and earth observation satellites contain optical instruments, which are aligned very precisely with a target, such as, a star in the case of astronomical satellites, or with a region of the earth. In order to produce precise data, from such satellites, deviations from predetermined trajectories must be minimized or at least the aiming trajectories must be reproducible. The alignment or position of the measuring instrument is subject to external disturbances such as gravitational variations, solar pressure, magnetic disturbances and several smaller effects, such as, Lorentzian forces. The bandwidth of the disturbances that are to be considered depends on the measurement precision to be obtained and in extreme cases, for example, for magnetic disturbances, this can amount to more that 100 Hz In order to control these disturbances, conventionally control elements such as gas jets reaction gears, (discrete or continuously operating) and the like are used. However, these, in turn, produce internal disruptions in the satellite itself. The bandwidth of these disturbances extends from the lowest frequencies in the sub-mHz region up to harmonics, which, for example, in the case of reaction gears, can have frequencies of several kHz. These mechanical disturbances are directly transferred to the measuring instrument via the satellite structure in conventional systems. Together with the external disturbances, they can lead to movements of the rigid body of the measuring instrument. In the case of large measuring instruments, such as telescopes, movements of the individual optical elements in the instrument relative to one another also must be taken into consideration in addition to the movement of the rigid body. These effects, in the case of optical instruments, lead to line-of-sight (LOS) movements and thus, among other things, to an undesired blurring of the images. In order to fulfill the goal of the mission, particularly in the case of high requirements for measurement precision, it is therefore necessary to minimize the influence of external and internal disturbances on the measuring device.
Disturbances of higher frequency are reduced according to the prior art by passive means for damping or isolating the sources of internal disruption from the satellite support of the utilized device. These passive means have only a limited lower bandwidth, and active means have recently been increasingly investigated for reducing structural vibrations, particularly for high-precision space interferometers. Further, magnetic means for isolating the utilized devices have been used in the case of microgravitational experiments. These active means suppress very well the effect of internal disturbances on the utilized devices in the high-frequency or intermediate-frequency ranges. However, they are not successful in the region of low frequencies, in which, however, a relatively rigid connection or coupling with the satellite structure still exists. The utilized device thus follows its movement. The magnitude of the satellite motion thus depends on the control precision that can be achieved and thus particularly both on the quality of the measurement signal obtained from the utilized device as well as the magnitude of external disturbances. Since the magnitude of the low-frequency satellite motion that occurs is a disadvantage for most applications, measures are necessary for limiting it.
It is therefore an object of this invention to provide a positioning system for high-precision satellite measuring instruments, which can substantially uncouple the instrument both from internal as well as from external disturbances especially in the low-frequency range.
The object is achieved by a positioning system for a high precision measuring instrument on a satellite, in which the positioning system comprises a support structure fixed to the satellite, said precision measuring instrument being freely movable with at least one degree of freedom within a space provided in said support structure, and a first positioning device operatively coupling said precision measuring instrument and said support structure to inertially align said measuring instrument and a target and cause said support structure to follow movement of said measuring instrument.
A clear increase in precision when compared with conventional systems results when a contact-free positioning of the measuring instrument is provided inside the satellite support structure, which is achieved in a manner that inertial travel of the measuring instrument is no longer produced by the primary control components (gas jets, reaction jets, etc.) of the satellite, but is made relative to the satellite and its support structure for the instrument. Since the satellite represents the main sources of disturbance, for example, primarily by production of magnetic dipole moments these are now isolated from the instrument. Also, isolated from the instrument are various external disturbances, such as, time-variable radiation pressure of the sun. Thus, the positioning control means for the instrument can, according to the invention, operate with lower power and thus achieve greater precision. That is, in contrast with satellite systems designed according to the prior art, here the chain of action has been reversed. The measuring instrument not only produces the measuring signals itself necessary for its inertial alignment, but is inertially aligned itself also as a xe2x80x9csatellitexe2x80x9d within the satellite support structure by means of a suitable control system. The satellite support structure now follows the movements of the measuring instrument as a screen against external disturbances of the measuring instrument. The control precision required for the satellite support structure is produced from the control means for positioning the instrument, i.e., the greater the control region of the positioning instrument, the less precise the control precision of the satellite. The degrees of freedom for which a decoupling between the satellite support structure and the positioning instrument depends on the respective application. Thus, in the cases of a standard optical instrument, only a decoupling of the rotational degrees of freedom will be necessary, while the translational degrees of freedom can be coupled relatively rigidly to the satellite support structure.
In the case of very high requirements for measurement precision, the changes in gravitational force between the measuring instrument and the satellite support structure must be minimized, because these can lead to increased requirements for the precision of the external control means of the satellite support structure.
Due to the extensive isolation of the measuring instrument from environmental influences, the movement of the measuring instrument becomes much xe2x80x9csmootherxe2x80x9d, i.e., in particular, it has fewer low-frequency fluctuations than is the case of an instrument that is not decoupled. The measuring instrument can be controlled in a simple way and its inertial movement can also be better reconstructed on the ground.
The arrangement further clearly reduces the requirements for resolution capacity and freedom from disturbances of control systems in the satellite structure. In this way, it is also possible, depending on the individual case, to provide the use of conventional control components for the external control circuit of the satellite structure even in the case of very high requirements for the measurement precision of the measuring instrument.